diff --git a/src/mobility/hierarchical-mobility-model.cc b/src/mobility/hierarchical-mobility-model.cc
index 7a129f052..7896a1f82 100644
--- a/src/mobility/hierarchical-mobility-model.cc
+++ b/src/mobility/hierarchical-mobility-model.cc
@@ -52,15 +52,46 @@ HierarchicalMobilityModel::HierarchicalMobilityModel ()
 void 
 HierarchicalMobilityModel::SetChild (Ptr<MobilityModel> model)
 {
+  Ptr<MobilityModel> oldChild = m_child;
+  Vector pos;
+  if (m_child)
+    {
+      pos = GetPosition ();
+      m_child->TraceDisconnectWithoutContext ("CourseChange", MakeCallback (&HierarchicalMobilityModel::ChildChanged, this));
+    }
   m_child = model;
   m_child->TraceConnectWithoutContext ("CourseChange", MakeCallback (&HierarchicalMobilityModel::ChildChanged, this));
+
+  // if we had a child before, then we had a valid position before;
+  // try to preserve the old absolute position.
+  if (oldChild)
+    {
+      SetPosition (pos);
+    }
 }
 
 void 
 HierarchicalMobilityModel::SetParent (Ptr<MobilityModel> model)
 {
+  Vector pos;
+  if (m_child)
+    {
+      pos = GetPosition ();
+    }
+  if (m_parent)
+    {
+      m_parent->TraceDisconnectWithoutContext ("CourseChange", MakeCallback (&HierarchicalMobilityModel::ParentChanged, this));
+    }
   m_parent = model;
-  m_parent->TraceConnectWithoutContext ("CourseChange", MakeCallback (&HierarchicalMobilityModel::ParentChanged, this));
+  if (m_parent)
+    {
+      m_parent->TraceConnectWithoutContext ("CourseChange", MakeCallback (&HierarchicalMobilityModel::ParentChanged, this));
+    }
+  // try to preserve the old position across parent changes
+  if (m_child)
+    {
+      SetPosition (pos);
+    }
 }
 
 
@@ -79,6 +110,10 @@ HierarchicalMobilityModel::GetParent (void) const
 Vector
 HierarchicalMobilityModel::DoGetPosition (void) const
 {
+  if (!m_parent)
+    {
+      return m_child->GetPosition ();
+    }
   Vector parentPosition = m_parent->GetPosition ();
   Vector childPosition = m_child->GetPosition ();
   return Vector (parentPosition.x + childPosition.x,
@@ -88,27 +123,41 @@ HierarchicalMobilityModel::DoGetPosition (void) const
 void 
 HierarchicalMobilityModel::DoSetPosition (const Vector &position)
 {
-  if (m_parent == 0 || m_child == 0)
+  if (m_child == 0)
     {
       return;
     }
   // This implementation of DoSetPosition is really an arbitraty choice.
   // anything else would have been ok.
-  Vector parentPosition = m_parent->GetPosition ();
-  Vector childPosition (position.x - parentPosition.x,
-			  position.y - parentPosition.y,
-			  position.z - parentPosition.z);
-  m_child->SetPosition (childPosition);
+  if (m_parent)
+    {
+      Vector parentPosition = m_parent->GetPosition ();
+      Vector childPosition (position.x - parentPosition.x,
+                            position.y - parentPosition.y,
+                            position.z - parentPosition.z);
+      m_child->SetPosition (childPosition);
+    }
+  else
+    {
+      m_child->SetPosition (position);
+    }
 }
 Vector
 HierarchicalMobilityModel::DoGetVelocity (void) const
 {
-  Vector parentSpeed = m_parent->GetVelocity ();
-  Vector childSpeed = m_child->GetVelocity ();
-  Vector speed (parentSpeed.x + childSpeed.x,
-               parentSpeed.y + childSpeed.y,
-               parentSpeed.z + childSpeed.z);
-  return speed;
+  if (m_parent)
+    {
+      Vector parentSpeed = m_parent->GetVelocity ();
+      Vector childSpeed = m_child->GetVelocity ();
+      Vector speed (parentSpeed.x + childSpeed.x,
+                    parentSpeed.y + childSpeed.y,
+                    parentSpeed.z + childSpeed.z);
+      return speed;
+    }
+  else
+    {
+      return m_child->GetVelocity ();
+    }
 }
 
 void 
diff --git a/src/mobility/hierarchical-mobility-model.h b/src/mobility/hierarchical-mobility-model.h
index f2d1cda70..5ef664863 100644
--- a/src/mobility/hierarchical-mobility-model.h
+++ b/src/mobility/hierarchical-mobility-model.h
@@ -27,8 +27,31 @@ namespace ns3 {
 /**
  * \brief a hierachical mobility model.
  * 
- * This model allows you to specify the position of a
- * child object relative to a parent object.
+ * This model allows you to specify the position of a child object
+ * relative to a parent object.
+ * 
+ * Basically this is a mobility model that combines two other mobility
+ * models: a "parent" model and a "child" model.  The position of the
+ * hierarchical model is always the vector sum of the parent + child
+ * positions, so that if the parent model "moves", then this model
+ * will report an equal relative movement.  Useful, for instance, if
+ * you want to simulate a node inside another node that moves, such as
+ * a vehicle.
+ *
+ * Setting the position on this model is always done using world
+ * absolute coordinates, and it changes only the child mobility model
+ * position, never the parent.  The child mobility model always uses a
+ * coordinate sytem relative to the parent model position.
+ *
+ * @note: as a special case, the parent model may be NULL, which is
+ * semantically equivalent to having a ConstantPositionMobilityModel
+ * as parent positioned at origin (0,0,0).  In other words, setting
+ * the parent model to NULL makes the child model and the hierarchical
+ * model start using world absolute coordinates.
+ *
+ * @warning: changing the parent/child mobility models in the middle
+ * of a simulation will probably not play very well with the
+ * ConfigStore APIs, so do this only if you know what you are doing.
  */
 class HierarchicalMobilityModel : public MobilityModel
 {
@@ -52,14 +75,26 @@ public:
    * position by the child mobility model.
    */
   Ptr<MobilityModel> GetParent (void) const;
+  /**
+   * Sets the child mobility model to a new one.  If before there
+   * already existed a child model, then the child mobility model
+   * current position is also modified to ensure that the composite
+   * position is preserved.
+   */
+  void SetChild (Ptr<MobilityModel> model);
+  /**
+   * Sets the parent mobility model to a new one.  If before there
+   * already existed a child model, then the child mobility model
+   * current position is also modified to ensure that the composite
+   * position is preserved.
+   */
+  void SetParent (Ptr<MobilityModel> model);
   
 private:
   virtual Vector DoGetPosition (void) const;
   virtual void DoSetPosition (const Vector &position);
   virtual Vector DoGetVelocity (void) const;
 
-  void SetChild (Ptr<MobilityModel> model);
-  void SetParent (Ptr<MobilityModel> model);
   void ParentChanged (Ptr<const MobilityModel> model);
   void ChildChanged (Ptr<const MobilityModel> model);